1. Field of the Invention
The present invention relates to a hydraulic control system for an automatic transmission, particularly to a hydraulic control system for 4-speed automatic transmission which can increase power train efficiency by controlling line pressure and can improve the responsiveness of shifting time during gear shifting.
2. Description of Prior Art
A conventional vehicle automatic transmission has a torque converter, a multiple stage transmission gear mechanism connected with the torque converter, and frictional elements actuated by hydraulic pressure for selecting one of the gear ratios of the transmission gear mechanism in accordance with vehicle operating conditions.
The hydraulic control system provides actuating hydraulic pressure pressurized by a fluid pump and required to operate the frictional elements and control valves.
A commonly used automatic transmission of a vehicle has a fluid torque converter which generally includes a pump impeller connected to an engine output shaft to be driven thereby, a turbine runner connected to an input shaft of the transmission, and a stator disposed between the pump impeller and the turbine runner, so that hydraulic fluid is circulated by the engine driven pump impeller through the turbine runner with the aid of the stator which functions to deflect the hydraulic fluid from the turbine runner to a direction where the fluid flow does not disturb the rotation of the pump impeller when the fluid flows from the turbine runner into the pump impeller.
The automatic shift is achieved by the operation of frictional elements such as clutches or kick-down brakes at each gear ratios. Also, a manual valve, the ports of which are converted by selecting a position of a selector lever, is designed to receive fluid from a fluid pump and to supply the fluid to a shift control valve. In case of a 4-speed automatic transmission, the shift control valve has a plurality of ports controlled by an electronic control system.
One example of a hydraulic pressure control system of an automatic transmission for a vehicle is described in FIG. 10, which shows a circuit diagram of a conventional hydraulic control system comprising a torque converter 1 attached to an engine through the engine flexible plate and rotating at engine speed for transmitting power of the engine to an input shaft of the transmission gear mechanism, a damper clutch control valve 2 for controlling the application and release of a damper clutch to increase the power train efficiency inside the torque converter 1, a regulator valve 5 for regulating the output hydraulic pressure of the fluid pump 4 according to the automatic transmission requirements, and a reducing valve 6 for regulating the stable supply of hydraulic pressure to a solenoid valve and the damper clutch control valve 2.
A manual valve 7, which is connected to an outlet of the fluid pump 4 and is provided with the hydraulic pressure, is designed to deliver line pressure to the regulator valve 5 and a shift control valve. The manual valve 7 is changed in position (P,R,N,D,2,L) by a shift lever as shown in FIG. 10.
A shift control valve 8, which is operated in response to two shift control solenoid valves A and B controlled by a transmission control unit (not shown) is designed to transmit the hydraulic pressure selectively through a first-second speed shift valve 9, an end clutch valve 10, a second-third and third-fourth speed shift valve 11, and a rear clutch exhaust valve 12 to a front clutch 13, a rear clutch 14, a low-reverse brake 15, a kickdown servo brake 16, an end clutch 17, and the like. An N-D control valve 18 to reduce the impact caused by shifting the selector lever from N range to D range is connected to the rear clutch 14. An N-R control valve 19 to reduce the impact caused by shifting the selector lever from N range to R range is connected to the low-reverse brake 15 via the first-second speed shift valve.
Also, a pressure control solenoid valve 20 is connected to a pressure control valve 21 to reduce the shock produced by the control at the time of shifting.
In such a conventional hydraulic circuit, two shift control solenoid valves A and B to control the positions of the valve spool in the shift control valve 8 send the hydraulic pressure produced by the fluid pump 4 to a first line D1, a second line D2, a third line D3, and a fourth line D4. When manual valve 7 is shifted to the R range, the hydraulic pressure is supplied through a reverse line R1. Following is the brief description of shifting operation in the system.
When D range is selected by a shift lever, hydraulic pressure generated by the fluid pump 4 is supplied to the manual valve 7 via a line L1, and supplied to the shift control valve 8 and the first line D1 via a line L2.
At a first speed of "D" range, both of the shift control solenoid valves A and B are controlled to be "ON" by the transmission control unit (`TCU`), and therefore the hydraulic pressure passing through the shift control valve 8 is exhausted to effect no change on the position of the valve spool. At the same time, the TCU makes the pressure control solenoid valve 20 switch "ON", and some of the hydraulic pressure returning via the reducing valve 6 is exhausted.
Since the hydraulic pressure in the first line D1 is not supplied to the first-second speed shift valve 9, the hydraulic pressure is supplied to the rear clutch 14 via the rear clutch exhaust valve 12 to actuate the same.
At a second speed of "D" range, the TCU makes the shift control solenoid valves A switch "OFF", and the hydraulic pressure is exhausted toward the shift control solenoid valve B to move the valve spool and a plug of the shift control valve 8 rightward so that the hydraulic pressure from the manual valve 7 is supplied to the line D2.
Accordingly, the hydraulic pressure in the line D2 is supplied to the left side of the first-second speed shift valve 9 to push the valve spool rightward in order to provide for a third speed. At this moment, the pressure control solenoid valve 20 is "OFF" state to keep the hydraulic pressure from being exhausted, and thus the hydraulic pressure from the fluid pump 4 is supplied to the left side of the pressure control valve 21 via the reducing valve 6 and a hydraulic line L3 to push the valve plug therein rightward. Accordingly, the hydraulic pressure passing through the first line D1 returns to the first-second speed shift valve 9 via the N-D control valve 18.
Because the valve spool of the first-second speed shift valve 9 is pushed rightward, the hydraulic pressure which has passed through the N-D control valve 18 is supplied to the kick-down servo brake 16 to actuate the same, and the second speed is accomplished thereby.
At a third speed of "D" range, because TCU makes both of the shift control solenoid valves A and B switch "OFF", the hydraulic pressure is kept from being exhausted. And the valve spool of the shift control valve 8 is moved rightward further, and the valve plug remains stopped.
At this state, because the second and third lines D2 and D3 are opened simultaneously, the hydraulic pressure passing through the second line D3 and being supplied to the right side of the end clutch valve 10 pushes the valve plug leftward and gets into the end clutch 17 to actuate the same.
The hydraulic pressure passing through the first-second speed shift valve 9 via the pressure control valve 21 passes through the second-third and fourth-third speed shift valve 11. Then, a portion of the hydraulic pressure acts to release the kick-down servo brake 16 and another portion of the hydraulic pressure acts to actuate the front clutch 13.
At this moment, the servo brake 16 at the second speed state is released by means of the hydraulic pressure supplied to the line connected to the front clutch 13.
At a fourth speed of "D" range, because TCU makes only the shift control solenoid valve B switch "OFF", the valve spool of the shift control valve 8 is moved to the right farther than it is at the third speed to open the fourth line D4.
Then the hydraulic pressure supplied to the left side of the rear clutch exhaust valve 12 pushes the valve spool rightward to cut off supplying the hydraulic pressure for actuating the front clutch 13 and the hydraulic pressure for releasing the kick-down servo brake 16. Accordingly, the kickdown servo brake 16 is actuated again automatically, the end clutch 17 at the third speed state is actuated also, and the fourth speed is accomplished thereby.
When the shift selector lever is at the R range, the hydraulic pressure from the manual valve 7 is supplied to the right side of the second-third and fourth-third speed shift valve 11 via the rear clutch exhaust valve 12 to push the valve spool leftward, and thus the hydraulic pressure from the manual valve 7 is supplied to the front clutch 13 and the low-reverse brake 15 and acts to release the kick-down servo brake 16 to reverse the vehicle.
In the above-mentioned automatic transmission control system, because the speed is shifted successively from the first speed to the fourth speed and the same hydraulic pressure from the fluid pump is supplied in two modes, or "DRIVE" and "REVERSE" ranges, even in high speed, it is impossible to be converted to lower hydraulic pressure, which leads to waste of power efficiency. In a conventional hydraulic pressure control system for controlling the forward four speeds it has not been possible to make a skip shifting, which results in a slow response of a gear shifting during high speed drive. Further, since control hydraulic pressure for controlling the pressure control valve is supplied later than actuating hydraulic pressure for frictional elements, which also results in a slow response of gear shifting.